html_prettify.rb

# frozen_string_literal: true

# heavily based off
# https://github.com/vmg/redcarpet/blob/master/ext/redcarpet/html_smartypants.c
# and
# https://github.com/jmcnevin/rubypants/blob/master/lib/rubypants/core.rb
# 99% of the code here is by Jeremy McNevin
#
# This Source File is available under BSD/MIT license as well as standard GPL
#

class HtmlPrettify < String
  def self.render(html)
    new(html).to_html
  end

  # Create a new RubyPants instance with the text in +string+.
  #
  # Allowed elements in the options array:
  #
  # 0  :: do nothing
  # 1  :: enable all, using only em-dash shortcuts
  # 2  :: enable all, using old school en- and em-dash shortcuts (*default*)
  # 3  :: enable all, using inverted old school en and em-dash shortcuts
  # -1 :: stupefy (translate HTML entities to their ASCII-counterparts)
  #
  # If you don't like any of these defaults, you can pass symbols to change
  # RubyPants' behavior:
  #
  # <tt>:quotes</tt>        :: quotes
  # <tt>:backticks</tt>     :: backtick quotes (``double'' only)
  # <tt>:allbackticks</tt>  :: backtick quotes (``double'' and `single')
  # <tt>:dashes</tt>        :: dashes
  # <tt>:oldschool</tt>     :: old school dashes
  # <tt>:inverted</tt>      :: inverted old school dashes
  # <tt>:ellipses</tt>      :: ellipses
  # <tt>:convertquotes</tt> :: convert <tt>&quot;</tt> entities to
  #                            <tt>"</tt>
  # <tt>:stupefy</tt>       :: translate RubyPants HTML entities
  #                            to their ASCII counterparts.
  #
  # In addition, you can customize the HTML entities that will be injected by
  # passing in a hash for the final argument.  The defaults for these entities
  # are as follows:
  #
  # <tt>:single_left_quote</tt>  :: <tt>&#8216;</tt>
  # <tt>:double_left_quote</tt>  :: <tt>&#8220;</tt>
  # <tt>:single_right_quote</tt> :: <tt>&#8217;</tt>
  # <tt>:double_right_quote</tt> :: <tt>&#8221;</tt>
  # <tt>:em_dash</tt>            :: <tt>&#8212;</tt>
  # <tt>:en_dash</tt>            :: <tt>&#8211;</tt>
  # <tt>:ellipsis</tt>           :: <tt>&#8230;</tt>
  # <tt>:html_quote</tt>         :: <tt>&quot; </tt>
  #
  def initialize(string, options = [2], entities = {})
    super string

    @options = [*options]
    @entities = default_entities.update(entities)
  end

  # Apply SmartyPants transformations.
  def to_html
    do_quotes = do_backticks = do_dashes = do_ellipses = nil

    if @options.include?(0)
      # Do nothing.
      return self
    elsif @options.include?(1)
      # Do everything, turn all options on.
      do_quotes = do_backticks = do_ellipses = true
      do_dashes = :normal
    elsif @options.include?(2)
      # Do everything, turn all options on, use old school dash shorthand.
      do_quotes = do_backticks = do_ellipses = true
      do_dashes = :oldschool
    elsif @options.include?(3)
      # Do everything, turn all options on, use inverted old school
      # dash shorthand.
      do_quotes = do_backticks = do_ellipses = true
      do_dashes = :inverted
    elsif @options.include?(-1)
      do_stupefy = true
    else
      do_quotes      = @options.include?(:quotes)
      do_backticks   = @options.include?(:backticks)
      do_backticks   = :both if @options.include?(:allbackticks)
      do_dashes      = :normal if @options.include?(:dashes)
      do_dashes      = :oldschool if @options.include?(:oldschool)
      do_dashes      = :inverted if @options.include?(:inverted)
      do_ellipses    = @options.include?(:ellipses)
      do_stupefy     = @options.include?(:stupefy)
    end

    # Parse the HTML
    tokens = tokenize

    # Keep track of when we're inside <pre> or <code> tags.
    in_pre = false

    # Here is the result stored in.
    result = +""

    # This is a cheat, used to get some context for one-character
    # tokens that consist of just a quote char. What we do is remember
    # the last character of the previous text token, to use as context
    # to curl single- character quote tokens correctly.
    prev_token_last_char = nil

    tokens.each do |token|
      if token.first == :tag
        result << token[1]
        if token[1] =~ %r!<(/?)(?:pre|code|kbd|script|math)[\s>]!
          in_pre = ($1 != "/")  # Opening or closing tag?
        end
      else
        t = token[1]

        # Remember last char of this token before processing.
        last_char = t[-1].chr

        unless in_pre

          t.gsub!("&#39;", "'")
          t.gsub!("&quot;", '"')

          if do_dashes
            t = educate_dashes t            if do_dashes == :normal
            t = educate_dashes_oldschool t  if do_dashes == :oldschool
            t = educate_dashes_inverted t   if do_dashes == :inverted
          end

          t = educate_ellipses t  if do_ellipses

          t = educate_fractions t

          # Note: backticks need to be processed before quotes.
          if do_backticks
            t = educate_backticks t
            t = educate_single_backticks t  if do_backticks == :both
          end

          if do_quotes
            if t == "'"
              # Special case: single-character ' token
              if prev_token_last_char =~ /\S/
                t = entity(:single_right_quote)
              else
                t = entity(:single_left_quote)
              end
            elsif t == '"'
              # Special case: single-character " token
              if prev_token_last_char =~ /\S/
                t = entity(:double_right_quote)
              else
                t = entity(:double_left_quote)
              end
            else
              # Normal case:
              t = educate_quotes t
            end
          end

          t = stupefy_entities t  if do_stupefy
        end

        prev_token_last_char = last_char
        result << t
      end
    end

    # Done
    result
  end

  protected

  # The string, with each instance of "<tt>--</tt>" translated to an
  # em-dash HTML entity.
  #
  def educate_dashes(str)
    str.
      gsub(/--/, entity(:em_dash))
  end

  # The string, with each instance of "<tt>--</tt>" translated to an
  # en-dash HTML entity, and each "<tt>---</tt>" translated to an
  # em-dash HTML entity.
  #
  def educate_dashes_oldschool(str)
    str.
      gsub(/---/, entity(:em_dash)).
      gsub(/--/,  entity(:en_dash))
  end

  # Return the string, with each instance of "<tt>--</tt>" translated
  # to an em-dash HTML entity, and each "<tt>---</tt>" translated to
  # an en-dash HTML entity. Two reasons why: First, unlike the en- and
  # em-dash syntax supported by +educate_dashes_oldschool+, it's
  # compatible with existing entries written before SmartyPants 1.1,
  # back when "<tt>--</tt>" was only used for em-dashes.  Second,
  # em-dashes are more common than en-dashes, and so it sort of makes
  # sense that the shortcut should be shorter to type. (Thanks to
  # Aaron Swartz for the idea.)
  #
  def educate_dashes_inverted(str)
    str.
      gsub(/---/, entity(:en_dash)).
      gsub(/--/,  entity(:em_dash))
  end

  # Return the string, with each instance of "<tt>...</tt>" translated
  # to an ellipsis HTML entity. Also converts the case where there are
  # spaces between the dots.
  #
  def educate_ellipses(str)
    str.
      gsub('...',   entity(:ellipsis)).
      gsub('. . .', entity(:ellipsis))
  end

  # Return the string, with "<tt>``backticks''</tt>"-style single quotes
  # translated into HTML curly quote entities.
  #
  def educate_backticks(str)
    str.
      gsub("``", entity(:double_left_quote)).
      gsub("''", entity(:double_right_quote))
  end

  # Return the string, with "<tt>`backticks'</tt>"-style single quotes
  # translated into HTML curly quote entities.
  #
  def educate_single_backticks(str)
    str.
      gsub("`", entity(:single_left_quote)).
      gsub("'", entity(:single_right_quote))
  end

  def educate_fractions(str)
    str.gsub(/(\s+|^)(1\/4|1\/2|3\/4)([,.;\s]|$)/) do
      frac =
        if $2 == "1/2".freeze
          entity(:frac12)
        elsif $2 == "1/4".freeze
          entity(:frac14)
        elsif $2 == "3/4".freeze
          entity(:frac34)
        end
      "#{$1}#{frac}#{$3}"
    end
  end

  # Return the string, with "educated" curly quote HTML entities.
  #
  def educate_quotes(str)
    punct_class = '[!"#\$\%\'()*+,\-.\/:;<=>?\@\[\\\\\]\^_`{|}~]'

    # normalize html
    str = str.dup
    # Special case if the very first character is a quote followed by
    # punctuation at a non-word-break. Close the quotes by brute
    # force:
    str.gsub!(/^'(?=#{punct_class}\B)/,
              entity(:single_right_quote))
    str.gsub!(/^"(?=#{punct_class}\B)/,
              entity(:double_right_quote))

    # Special case for double sets of quotes, e.g.:
    #   <p>He said, "'Quoted' words in a larger quote."</p>
    str.gsub!(/"'(?=\w)/,
              "#{entity(:double_left_quote)}#{entity(:single_left_quote)}")
    str.gsub!(/'"(?=\w)/,
              "#{entity(:single_left_quote)}#{entity(:double_left_quote)}")

    # Special case for decade abbreviations (the '80s):
    str.gsub!(/'(?=\d\ds)/,
              entity(:single_right_quote))

    close_class = %![^\ \t\r\n\\[\{\(\-]!
    dec_dashes = "#{entity(:en_dash)}|#{entity(:em_dash)}"

    # Get most opening single quotes:
    str.gsub!(/(\s|&nbsp;|=|--|&[mn]dash;|#{dec_dashes}|&#x201[34];)'(?=\w)/,
             '\1' + entity(:single_left_quote))

    # Single closing quotes:
    str.gsub!(/(#{close_class})'/,
              '\1' + entity(:single_right_quote))
    str.gsub!(/'(\s|s\b|$)/,
              entity(:single_right_quote) + '\1')

    # Any remaining single quotes should be opening ones:
    str.gsub!(/'/,
              entity(:single_left_quote))

    # Get most opening double quotes:
    str.gsub!(/(\s|&nbsp;|=|--|&[mn]dash;|#{dec_dashes}|&#x201[34];)"(?=\w)/,
             '\1' + entity(:double_left_quote))

    # Double closing quotes:
    str.gsub!(/(#{close_class})"/,
              '\1' + entity(:double_right_quote))
    str.gsub!(/"(\s|s\b|$)/,
              entity(:double_right_quote) + '\1')

    # Any remaining quotes should be opening ones:
    str.gsub!(/"/,
              entity(:double_left_quote))

    str
  end

  # Return the string, with each RubyPants HTML entity translated to
  # its ASCII counterpart.
  #
  # Note: This is not reversible (but exactly the same as in SmartyPants)
  #
  def stupefy_entities(str)
    new_str = str.dup

    {
      en_dash: '-',
      em_dash: '--',
      single_left_quote: "'",
      single_right_quote: "'",
      double_left_quote: '"',
      double_right_quote: '"',
      ellipsis: '...'
    }.each do |k, v|
      new_str.gsub!(/#{entity(k)}/, v)
    end

    new_str
  end

  # Return an array of the tokens comprising the string. Each token is
  # either a tag (possibly with nested, tags contained therein, such
  # as <tt><a href="<MTFoo>"></tt>, or a run of text between
  # tags. Each element of the array is a two-element array; the first
  # is either :tag or :text; the second is the actual value.
  #
  # Based on the <tt>_tokenize()</tt> subroutine from Brad Choate's
  # MTRegex plugin.  <http://www.bradchoate.com/past/mtregex.php>
  #
  # This is actually the easier variant using tag_soup, as used by
  # Chad Miller in the Python port of SmartyPants.
  #
  def tokenize
    tag_soup = /([^<]*)(<[^>]*>)/

    tokens = []

    prev_end = 0

    scan(tag_soup) do
      tokens << [:text, $1]  if $1 != ""
      tokens << [:tag, $2]
      prev_end = $~.end(0)
    end

    if prev_end < size
      tokens << [:text, self[prev_end..-1]]
    end

    tokens
  end

  def default_entities
    {
      single_left_quote: "&lsquo;",
      double_left_quote: "&ldquo;",
      single_right_quote: "&rsquo;",
      double_right_quote: "&rdquo;",
      em_dash: "&mdash;",
      en_dash: "&ndash;",
      ellipsis: "&hellip;",
      html_quote: "&quot;",
      frac12: "&frac12;",
      frac14: "&frac14;",
      frac34: "&frac34;",
    }
  end

  def entity(key)
    @entities[key]
  end

end